Besides its high hardness and thermal conductivity, diamond also has many other excellent properties, such as a high optical transmittance and a wide bandgap, and is therefore widely used as a material for various tools, optical components, semiconductors, and electronic components, and its importance is only expected to grow in the future. In the past, naturally produced diamond has been used in industrial applications, but since natural diamond has a quite-variable quality, more and more manmade diamond is being used an industrial setting. Diamond single crystals today are synthesized industrially under high temperature and pressure (several thousand degrees centigrade, and several tens of thousands of atmospheres). Super-high-pressure vessels that can withstand such high temperatures and pressures are extremely expensive, and there is a limit to their size, which imposes a limit to how large a single crystal can be synthesized by high-temperature, high-pressure methods. Ib-type diamond which contains nitrogen (N) as an impurity and is yellow in color has been synthesized by high-temperature, high-pressure methods and marketed in a diameter of about 1 cm, but this approximate size is thought to be the limit. IIa-type diamond which contains few impurities and is colorless and transparent can be mass-produced industrially in a size of only about a few millimeters.
Meanwhile, another method that has been established as a diamond synthesis method alongside the high-temperature, high-pressure method is vapor phase synthesis. With this method, diamond with a relatively large surface area of from a few centimeters up to 10 cm, or even larger, can be manufactured artificially, but the product is usually a polycrystalline film. However, of the many applications of diamond, when the product is used for semiconductor substrates, optical components, or ultra-precision tools that require particularly smooth surfaces, it is necessary to use single-crystal diamond. In view of this, methods for obtaining single crystal diamond by epitaxial growth by vapor phase synthesis have been studied in the past.
Epitaxial growth is generally broken down into homoepitaxial growth in which the substance to be grown is grown on a substrate of the same type, and heteroepitaxial growth in which the substance is growth on a different type of substrate. With heteroepitaxial growth, there have been reports dealing with cubic boron nitride (cBN), silicon carbide (SiC), silicon (Si), nickel, cobalt, and so forth (see Patent Documents 1 to 3 listed below), but since single crystals with good film quality cannot be obtained by heteroepitaxial growth, synthesis of single crystals by homoepitaxial growth is considered more effective. With homoepitaxial growth, it is possible to obtain large IIa diamond single crystals that are greater than IIa diamond obtained by a high-temperature, high-pressure method, by epitaxially growing high-purity diamond from the vapor phase on an Ib diamond substrate produced by high-temperature, high-pressure synthesis. It has also been reported that diamond having just small angle boundaries can be obtained by using a plurality of diamond substrates all oriented in the same crystal orientation, or using diamond particles and growing integrated diamond over these (see Patent Document 4).    Patent Document 1: Japanese Patent Publication 63-224225A    Patent Document 2: Japanese Patent Publication 2-233591A    Patent Document 3: Japanese Patent Publication 4-132687A    Patent Document 4: Japanese Patent Publication 3-75298A